Adenosine is a naturally occurring nucleoside, which exerts its biological effects by interacting with a family of adenosine receptors known as A1, A2A, A2B, and A3, all of which modulate important physiological processes. For example, A2A adenosine receptors modulate coronary vasodilation, A2B receptors have been implicated in mast cell activation, inflammation, asthma, vasodilation, regulation of cell growth, intestinal function, and modulation of neurosecretion (See Adenosine A2B Receptors as Therapeutic Targets, Drug Dev Res 45:198; Feoktistov et al., Trends Pharmacol Sci 19:148-153), and A3 adenosine receptors modulate cell proliferation processes.
Adenosine A2B receptors are ubiquitous, and regulate multiple biological activities. For example, adenosine binds to A2B receptors on endothelial cells, thereby stimulating angiogenesis. Adenosine also regulates the growth of smooth muscle cell populations in blood vessels. Adenosine stimulates A2B receptors on mast cells, thus modulating Type I hypersensitivity reactions. Adenosine also stimulates gastrosecretory activity by ligation with A2B in the intestine.
While many of these biological effects of adenosine are necessary to maintain normal tissue homeostasis, under certain physiological changes it is desirable to modulate its effects. For example, the binding of A2B receptors stimulates angiogenesis by promoting the growth of endothelial cells. Such activity is necessary in healing wounds, but the hyperproliferation of endothelial cells promotes diabetic retinopathy. Also, an undesirable increase in blood vessels occurs in neoplasia. Accordingly, inhibition of the binding of adenosine to A2B receptors in the endothelium will alleviate or prevent hypervasculation, thus preventing retinopathy and inhibiting tumor formation.
A2B receptors are found in the colon in the basolateral domains of intestinal epithelial cells, and when acted upon by the appropriate ligand act to increase chloride secretion, thus causing diarrhea, which is a common and potentially fatal complication of infectious diseases such as cholera and typhus. A2B antagonists can therefore be used to block intestinal chloride secretion, and are thus useful in the treatment of inflammatory gastrointestinal tract disorders, including diarrhea.
Insensitivity to insulin exacerbates diabetes and obesity. Insulin sensitivity is decreased by the interaction of adenosine with A2B receptors. Thus, blocking the adenosine A2B receptors of individuals with diabetes or obesity would benefit patients with these disorders. It has also been demonstrated that A2B-antagonists cause a reduction of blood glucose levels, and thus would be particularly useful in the treatment of type-II diabetes.
Another adverse biological effect of adenosine acting at the A2B receptor is the over-stimulation of cerebral IL-6, a cytokine associated with dementias and Alzheimer's disease. Inhibiting the binding of adenosine to A2B receptors would therefore mitigate those neurological disorders that are produced by IL-6.
Type I hypersensitivity disorders, such as asthma, hay fever, and atopic eczema, are stimulated by binding to A2B-receptors of mast cells. Therefore, blocking these adenosine receptors would provide a therapeutic benefit against such disorders.
There are several compounds presently used in the treatment of asthma. For example, theophylline is an effective antiasthmatic agent, even though it is a poor adenosine receptor antagonist. However, considerable plasma levels are needed for it to be effective. Additionally, theophylline has substantial side effects, most of which are due to its CNS action, which provide no beneficial effects in asthma, and to the fact that it non-specifically blocks all adenosine receptor subtypes.
Additionally adenosine treatment, such as inhaled adenosine (or adenosine monophosphate), provokes bronchoconstriction in asthmatics, but not in the normal population. This process is known to involve mast cell activation, in that it releases mast cell mediators, including histamine, PGD2-β-hexosaminidase and tryptase, and because it can be blocked by specific histamine H1 blockers and chromolyn sodium. Accordingly, there is an intrinsic difference in the way adenosine interacts with mast cells from asthmatics, and thus A2B antagonists are particularly useful in modulating mast cell function or in the activation of human lung cells.
Accordingly, it is desired to provide compounds that are potent A2B antagonists (i.e., compounds that inhibit the A2B adenosine receptor), fully or partially selective for the A2B receptor, useful in the treatment of various disease states related to modulation of the A2B receptor, for example cancer, rheumatoid arthritis, asthma, chronic obstructive pulmonary disorder, pulmonary fibrosis, and retinopathy.